The invention relates to a synthetic quartz glass preform and a device for producing the same.
In accordance with developments in the semiconductor industry, and utilization of the products from the semiconductor industry in various fields of application, as well as due to independent developments, in particular in the special fields of materials management and medicine, light sources with very high energy densities find application. Particularly, these are excimer laser with operation wavelengths of 248 nm and 193 nm. The optical components used thereby for imaging and directing the radiation, as well as the photomasks which exclusively consist of synthetic quartz glass or calcium fluoride have to satisfy the required optical quality and must not lose the same in continuous operation. The most important high-quality features of the optical components arid the most difficult ones to be set, are optical homogeneity and stability with respect to excimer laser irradiation in the deep ultra-violet light (DUV). Therefore, there was no lack of trials in the past to obtain such high-quality features permanently and reproducibly.
Hence, there is known a method for producing a homogeneous, striae free body of quartz glass from DE 42 04 406 A1, in which a rod-shaped initial body is twisted, multiply thermally remodeled in a mould of suitable foreign material and twisted again. In the EP 0 673 888 A1 this method is modified under avoidance of any contact with a foreign material in such a manner that a quartz glass body subsequently produced according to the method is optically homogeneous in three directions and additionally is stable with respect to excimer laser radiation. However, EP 0 673 888 A1 does not teach to which degree this stability is achieved. Additionally, the method is considerably time and cost consuming.
Synthetic quartz glass is characterized by having very good transmission in the deep range of ultra-violet light (DUV). When it is exposed to high energy short-wave radiation as, for example, provided by excimer lasers at 248 nm and 193 nm, photochemical reactions will result, which will lead to the formation of paramagnetic defects, the latter being responsible for the formation of absorption bands and the development of luminescence. The power of these photochemical reactions depends on intrinsic defects in the form of binding anomalies. The photochemical reactions are also intensified by contaminants in a network as given, for example, by atoms of transition metals and chlorine. Parallel to these photochemical reactions which impair the optical properties of the quartz glass, annealing processes take place for which an OH content and a content of free hydrogen in the quartz glass is of importance.
From the subsequently discussed prior art it is known to desensitize synthetic quartz glass to high-energy radiation in the DUV by the following measures, carried out individually or in combination: introducing molecular hydrogen into the quartz glass bulk, using particularly pure starting raw material, using chlorine free starting raw material, and doping the quartz glass with fluorine and others. The EP 0 483 752 A1 ( U.S. Pat. No. 5,410,428) reference relates to a synthetic silica glass with a content of molecular hydrogen of at least 5xc2x71016 molecules/cm3 which is manufactured by a process wherein a quartz glass body is exposed to a hydrogen atmosphere in a furnace at a high temperature and a high pressure for a defined time, until a desired hydrogen concentration has been established in its interior; subsequently the silica glass body is definedly cooled down to ambient temperature. This silica glass is known as being very stable against high-energy radiation in the DUV, although it has only been exposed to 2xc2x7106 laser shots. It is disadvantageous that an after treatment of the silica glass is necessary including extensive safety measures required thereto. Furthermore, the produced silica glass bodies exhibiting the desired properties may not be of very large volume.
The EP 0 525 984 A1 reference describes a method for producing quartz glass which is adapted to be exposed to an excimer laser irradiation. However, the resistance property of the same is only disclosed up to a laser shot rate of about 106 at an energy density of 200 mJ/cm2, a shot frequency of 100 Hz and a wavelength of xcex=193 nm. The method does not function without a specific homogenizing step which renders it expensive.
The patent specification EP 0 737 654 A1 relates to a synthetic quartz glass with a content of molecular hydrogen of at least 1018 molecules/cm3 and a low OH content of a maximum of 50 ppm, which at a temperature of maximally 500xc2x0 C. and under a high pressure is enriched with H2. The stability is specified with 1.3xc2x7107 laser shots at an energy density of 350 mJ/cm2, a shot frequency of 400 Hz and a wavelength of 248 nm, Also in this case, a subsequent treatment of the quartz glass is required, to which end a chlorine free raw material can be used.
In U.S. Pat. No. 5,364,433 a synthetic quartz glass suited for production of DUV-stepper lenses and a method for producing the same is disclosed. The quartz glass exhibits an OH content of 10-100 ppm, a chlorine content of maximally 200 ppm, a molecular hydrogen content of  less than 1016 molecules/cm3, a refractive index homogeneity of  greater than 5xc2x710xe2x88x926 and a strain of  greater than 5 nm/cm. The stability of this quartz glass against excimer laser irradiation at a low absorption is only disclosed up to a low 10 shot rate of 0.8xc2x7106 (energy density 200 mJ/cm2, shot frequency 100 Hz, xcex=193 nm). The comparatively low stability is explained in that a dehydration step provided for in the manufacturing process leads to an increase of the Cl content which, in turn, reduces the DUV stability. An additionally provided homogenizing step renders the method more expensive.
A substrate plate for photomasks which shows a H2 content between 1017 and 1019 molecules/cm3 is disclosed in EP 0 636 586 A1. This solution is little or not at all suited for the production of imaging optical members in the DUV range, which are subject to considerably higher requirements with regard to the transmission and the optical homogeneity than photomasks.
U.S. Pat. No. 5,086,352 discloses optical components made of synthetic quartz glass which can be employed in DUV excimer laser irradiation and a method for producing the same. The optical components exhibit an OH concentration of at least 100 ppm and a doped hydrogen concentration of at least 5xc2x71016 molecules/cm3 (and an amount of 1xc2x71020 molecules/cm3 released at degassing, respectively,) and are free from stratification in at least one direction. 50 ppb are given for the chemical purity of the component in the most pretentious case for Na, K and LI, and 10 ppb for Mg, Ca, Ti, Cr, Fe, Ni, and Cu. The preforms of the optical components are characterized in that there is no stratification parallel to the incident light, that the OH concentration rises from a central minimum to a maximum without a point of inflection, that in the range between minimum and maximum the refractive index inhomogeneity is 2xc2x710xe2x88x926 or lower, and in that there exists a hydrogen doping. Such a preform shows profiles of the OH concentration, of the Cl concentration and of a fictitious temperature which are to be adjusted for obtaining a high refractive index of homogeneity. The method for producing the optical components comprises, in each case, steps for removing stratifications and for doping with hydrogen which renders the entire production process complicated and expensive. Moreover, the stability is only given up to a comparatively low laser shot rate of 107 (energy densities: 400 and 100 mJ/cm2, respectively, shot frequencies: 100 Hz, xcex: 248 and 193 nm, respectively,).
U.S. Pat. No. 5,325,230 is based on U.S. Pat. No. 5,086,352 and additionally requires for the optical component of synthetic quartz glass an absence of oxygen defects and a strain birefringence, which has to be  less than 5 nm/cm. The OH concentration distribution is axially symmetrical. Also in this case, stratifications have to be removed and doping with hydrogen has to be carried out in the course of manufacturing the optical components. Considerable and expensive efforts have to be made to obtain a high purity of the quartz glass, which also finds expression in that special measures have to be taken for storing the basic materials.
In the EP 0 747 327 A1 reference, a method for the heat treatment and consolidation of a quartz glass preform is described whereby a reduction of the laser induced defects in the quartz glass is asserted to be obtained. There is nothing reported of the refractive index homogeneity, of the form and mass of the bodies to be produced, of a feasible application of the produced quartz glass under extreme conditions. The represented increases in absorption at 248 nm and 193 nm, respectively, are only acceptable up to few million shots.
EP 0 622 340 A1 discloses an improved method for producing a body of synthetic silica glass. A burner comprising at least five nozzles is supplied with fuel gas in such a manner that the produced synthetic silica glass shows an OH content optimized compared to the H2 content. There is nothing reported with respect to the DUV stability and the refractive index homogeneity. For obtaining OH contents above 1150 ppm, this procedure is unstable with respect to the attainable growth behavior.
In EP 0720 969 A1, a quartz glass, an optical component containing this quartz glass and a manufacturing process for the quartz glass are described. For the production of the preforms a downward directed burner. is employed. The stability of the quartz glass with respect to the excimer laser irradiation lies at a comparatively low shot rate of about 106. A Cl content of 10 ppm is achieved by an extremely low uneconomical raw material feed of 70 g/minxc2x7cm2 via the central nozzle of the burner. The OH concentration of the quartz glass substantially lies at only 900 ppm.
In EP 0 720 970 A1 there is described a quartz glass for photolithographic applications, an optical component containing said quartz glass, a photolithographic device containing said component, and a method for producing the quartz glass. There are conditions disclosed for producing a preform which can also be utilized in the DUV. However, the stability of the quartz glass against excimer laser irradiation is only represented up to 106 shots. The quartz glass is subjected to an F-doping which ensures, as known, low dispersion losses and has a favorable effect on the DUV stability. However, a high optical homogeneity of the melted quartz glass will not. be attainable owing to the F-doping. In the course of SiO2 deposit on places with the highest temperature, there also develop the highest OH concentration and the highest F concentration. Hence, an error is introduced which increases the gradient of the refractive index curve.
Finally, EP 0 735 006 describes a method for producing quartz glass in which the growth process of the quartz glass produced synthetically takes place in an upright direction. The process is controlled in such a manner that the stratification is adapted to take place only vertically to the growth direction of the preform.
By virtue of the present invention the disadvantages of the prior synthetic quartz glasses are obviated which, up to now, did not permit the utilization of the same in extreme applications in the DUV. Therefore, it is an object of the present invention to produce, under use of a flame hydrolysis technique, a synthetic quartz glass which meets highest requirements concerning stability with respect to excimer laser irradiation in the DUV at a high energy density and concerning optical homogeneity. It is a further object of present invention to provide a device which is particularly suited in the manufacture of the quartz glass and which renders the output of the manufacturing process maximal.
According to the present invention, the objects are realized by the characteristic features described herein. It is also feasible to employ radiation of other wavelengths, provided that the same lies under 250 nm. The excitation conditions can be varied; for example, a transmission reduction of xcex94T less than 0.05%, is obtained at a laser shot frequency (frequency) of  greater than 400 Hz, a laser shot rate (shot number) of  greater than 108, and an energy density  less than 25 mJ/cm2 with respect to the wavelength xcex1=248 nm. The transmission reduction corresponds to damage behavior for values stated herein. Hence, it lies within the scope of the invention. It can be generally stated that a varied reduction of the internal transmission takes place at a radiation variation, but an unchanged damage behavior. Damage behavior is to be understood by someone skilled in the art as a long-term damage, for example, a transmission variation of synthetic quartz glass under the effect of an excimer laser irradiation.
A core area of the preform extends over at least 50 to 90% of the preform diameter, which can amount to up to 18 cm and more. It shows neither an axial stratification nor a stratification at right angles to its direction of growth; its entire volume is free from stratifications. The growth range of the drum-shaped preform has an at least almost flat part close to the center, which substantially conforms to the core, and a peripheral part with a parabolic face which passes over into a cylindrical surface of the drum-shaped preform. The cross-sectional area of the preform which can be utilized for different purposes and in which the quality of the synthetic quartz meets the respective requirements is different. Thus, for example, it is sufficient when used in illumination systems for excimer laser, that the synthetic quartz glass has high stability and transmission at an adequate homogeneity. In this case, 70% to 90% of the inner cross-section of the preform can be utilized. When projection elements for directing high-energy laser irradiation are made from the preform, then under the same conditions a limitation to the inner 50 to 70% of the cross-section of the preform is necessary. Thereby, it is essential that across the inner cross-section of the preform not only high stability and transmission exist, but also high homogeneity; this means, however, that the OH content of the preform is constant to xc2x110 ppm over this inner cross-section. Advantageously, the OH content of the core area of the preform amounts to at least 1250 ppm at a tolerance of xc2x110 ppm. The Cl content of the same does not exceed 20 ppm and preferably is 5 to 15 ppm. The H2 content of the core area of the preform advantageously amount to  greater than 1xc2x71018 or molecules/cm3. A preform having the abovementioned parameters is, to a high degree, stable against high energy DUV irradiation, shows a high refractive index stability and is excellent for the production of optical members such as DUV stepper-lenses, directing members for laser beams, photomasks etc. At least over a part of the core area, the preform advantageously exhibits a refractive index homogeneity of  less than 0.5xc2x710xe2x88x926. Thereby, traces of contaminating elements (e.g. Cr, Co, Fe, Ni, Cu, V, Zn, Al, Li, K, Na) can be contained in the preform up to 500 ppb. The preform does not require any additional doping with H2, F and others, to render it serviceable for tasks in DUV excimer laser irradiation. Also a subsequent treatment of the synthetic quartz glass in a reducing atmosphere is not required. If necessary, it is advantageous to cut optical members out of the material of the core area.
A device for producing the preform comprises a substantially horizontal muffle with two differently sized arranged openings opposing each other, the larger of which is adapted for inserting the preform and the smaller one for inserting a burner, and an internal chamber which narrows from the larger opening to the smaller opening. The burner is provided with nozzles which are coaxially arranged to each other and to the burner axis, the centrally arranged nozzle discharges the basic material, for example, SiCl4 and O2 and the external nozzles the fuel gas, for example, H2 and O2, parallel to one another and to the burner axis. The narrowing substantially is a gradual one. Unlike similar prior devices, the muffle has neither an opening nor a bulge on its top-side. The overall length of the muffle is at least twice the size of the diameter of the vitreous preform. The almost planar leading face of the latter is preferably arranged in the center of the internal chamber: of the muffle. The muffle is preferably embodied in three layers in order to ensure a sufficient and constant internal temperature as well as a low heat emission. It is advantageous, when the distance of the substantially rotation-symmetrical preform surface relative to the internal limiting face is 5 to 100 mm depending on the flow conditions for the waste gas. Furthermore, it is advantageous, when the distance of the burner to the preform is 135 to 350 mm in dependence on the geometry of the burner nozzles and the flow of the fuel gases volume. As to the smaller opening, in which the burner is arranged for free movement, a diameter of 50 to 100 mm is to be recommended.
Due to the internal geometry of the muffle and the operation of the burner, the device of the present invention ensures that the preform is definedly distributed by the fuel gas as well as that preforms of principally optional lengths can be the melted on. No subsequent treatment (twisting, doping) is required for the preform. A modification of geometry in order to adapt the preform to the intended application can be combined with a heating of the preform. In spite of extreme process control the device permits melting preform masses of 50 kg and more, in a normal melting process, which are optically homogeneous and stable in the DUV against high-energy laser beams.